As other commenters noted, rear wheel regenerative braking doesn’t work very well. But there are more problems: most mid-drive e-bikes fundamentally can’t regeneratively brake at all: the rear hub freewheels and cannot drive the motor. Even ignoring that, the chain/belt frequently also can’t drive the motor because that would cause the pedals to drive the motor, and a lot of e-bikes are designed to be pleasant to ride with the motor off, and the rotor has rotational inertia and often has drag as well.